(1) Studies on a novel magnesium-dependent, calcium-inhibited protein phosphatase revealed that it can be activated by its protein activator about 360-fold with an activation constant of 0. 1 micromolar. The enzyme contains a single methionine and half of the amino acid residues are either acidic or basic. The possibility that the enzyme may be a tyrosine phosphatase has been ruled out. (2) Site-directed mutagenesis of Escherichia coli ribonucleotide reductase revealed that the carboxyl terminus of the B2 subunit is the key region involved in Bl-B2 subunit interaction and possibly electron transfer during catalysis. (3) Kinetic methods for differentiation of (a) simultaneous binding of two inhibitor molecules in linear or nonlinear competitive inhibition and (b) whether a low enzymatic activity association with a mutant enzyme is due to wild type contamination or residual intrinsic activity, have been developed.